1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to a pixel electrode structure of liquid crystal display device.
2. The Related Arts
Liquid crystal displays (LCDs) are one of the most commonly used flat panel displays. An LCD comprises a pair of substrates that are provided with field generating electrodes, such as pixel electrode and common electrode, and a liquid crystal (LC) layer interposed between the two substrates. When a voltage is applied to the field generating electrode to induce an electric field in the LC layer, the electric field determines the orientation of the LC molecules of the liquid crystal layer so as to adjust the polarization of the light incident to the liquid crystal layer to allow an image to be displayed by the LCD.
A so-called polymer stabilized vertical alignment (PSVA) technique has been developed in the industry and such a technique mixes monomer of a suitable concentration in a liquid crystal material and uniformly shakes. Afterwards, the mixed liquid crystal material is positioned in a heater to be heated until reaching a condition of isotropy. When the liquid crystal mixture is cooled down to the room temperature, the liquid crystal mixture returns to a nematic condition. Afterwards, the liquid crystal mixture is filled into a liquid crystal box and is subject to application of voltage. When the applied voltage makes the arrangement of liquid crystal molecules stable, the monomer is caused to perform polymerization by means of ultraviolet radiation or heating in order to form a polymer layer, so as to achieve the purpose of alignment stabilization.
FIG. 1 is a schematic view showing a conventional design of pixel electrode that is commonly used in regular VA (Vertical Alignment) mode liquid crystal display (LCD) devices.
As shown in FIG. 1, the regular VA mode LCD has pixel electrodes that are of snow flake like configuration, comprising a strip-like vertical main trunk, a strip-like horizontal main trunk, and strip-like branches that are set at predetermined angle with respect to the horizontal main trunk. Typically, the angles between the strip-like branches and the horizontal main trunk are ±45 degrees and ±135 degrees. Each of the strip-like branches is located on the same plane as the vertical main trunk and the horizontal main trunk. The vertical main trunk and the horizontal main trunk centrally and perpendicularly intersect each other. The term “central perpendicular intersection” as used herein refers to the vertical main trunk and the horizontal main trunk being perpendicular to each other and an area around a center of the perpendicular intersection being a central zone of a unit pixel electrode, the vertical main trunk and the horizontal main trunk equally dividing the whole pixel area into four domains, each domain being formed of flat-laying strip-like branches that show predetermined angles with respect to the vertical main trunk or the horizontal main trunk. In this way, an arrangement of snow flake like electrode that is of mirror symmetry in up-down direction and left-right direction is formed as shown in FIG. 1.
FIG. 2 is a schematic view illustrating inclination of liquid crystal with a voltage (around 0 to 4V, arrow indicating the application of voltage) applied to the structure of pixel electrode shown in FIG. 1.
As shown in FIG. 2, when a snow flake like electrode structure that is regularly adopted is energized, the orientation of liquid crystal is gradually inclined from outside of the pixel electrode toward inside of the pixel electrode and the angle of inclination is along the direction of the branches. In the four domains, the liquid crystal of the right upper domain is inclined in a direction of 45 degrees; the liquid crystal of the left upper domain is inclined in a direction of 135 degrees; the liquid crystal of the left lower domain is inclined in a direction of 225 degrees; and the liquid crystal of the right lower domain is inclined in a direction of 315 degrees. The inclination directions of the liquid crystals in all the domains point toward the central zone of the pixel electrode.
To increase transmittance of a panel, the structure of pixel electrode of the known technique makes the central zone symmetric. However, for a unit pixel electrode, since all liquid crystals are inclined at directions pointing toward the central zone of the pixel electrode, a swirl like liquid crystal rotation region is definitely formed in the main trunks, of which a specific simulation is shown in FIG. 3. FIG. 3 is a schematic view illustrating inclination of liquid crystal in a structure of pixel electrode causes formation of a swirl at the center. FIG. 4 is a schematic view of the structure of pixel electrode corresponding to FIG. 3 observed with a microscope, in which reference character F indicates the location where the swirl like liquid crystal rotation region caused by inclination of liquid crystal.
However, since the chance that the swirl like rotation region caused by inclination of liquid crystal occurs on a central zone of the main trunks or on an edge portion of the main trunks is substantially the same and thus, the swirl like rotation region may sometime occur on an edge portion of the main trunks of which a specific simulation is illustrated in FIG. 5. FIG. 5 is a schematic view illustrating a swirl caused by inclination of liquid crystal in a structure of pixel electrode is sideway shifted. Due to such a situation, once a pixel electrode is energized to drive reorientation of liquid crystal, if a swirl like liquid crystal rotation region F occurs on an edge portion of the main trunks, then non-uniformly displayed disclination lines may occur around the swirl like liquid crystal rotation region F, as specifically shown in the schematic view of observation with a microscope. FIG. 6 is a schematic view of the structure of pixel electrode corresponding to FIG. 5 observed with a microscope. In FIG. 6, the serration shown around the swirl like liquid crystal rotation region F is disclination. In the known technique illustrated in FIG. 6, the “disclination” phenomenon caused by the swirl liquid crystal rotation region occurs at edges of displaying sites of upper and lower displaying domains at one side portion of the unit pixel electrode and this makes a displaying area become a light-blocked area, leading to reduction of transmittance and poor quality of image displaying.